Reduction of aromatic nitrogen compounds



United States Patent REDUCTION OF AROMATIC NITROGEN COMPOUNDS Francis W. Cashion, Hamburg, N. Y., assignor to Allied Chemical & Dye Corporation, New York, N. Y., a corporation of New York No Drawing. Application May 26, 1952, Serial No. 290,089

20 Claims. (Cl. 260-205) This invention relates to improvements in the reduction of aromatic nitrogen compounds containing nitrogen in a reducible form as a nuclear substituent with metal alcoholates. It relates more particularly to a method of promoting said reduction of such aromatic nitrogen compounds, and especially of mononuclear aromatic nitrogen compounds of said type, in which the nitrogen is at a higher stage of oxidation than the hydrazo stage.

The alkaline reduction of aromatic nitrogen compounds containing nitrogen in a reducible form is well known. Thus it is known to reduce aromatic nitro compounds, aromatic nitroso compounds, aromatic azoxy compounds, aromatic azo compounds and aromatic hydroxylamino compounds by means of alkaline reducing agents.

It' is also known that the extent to which the reduction can be carried depends on the strength of the reducing agent and the severity of the reduction conditions. For example, in the reduction of nitrobenzene with a metal alc'oholate', and especially an alkali metal alcoholate, the reduction ordinarily does not go beyond the azoxy stage. The reduction is usually carried out by heating nitrobenzene with alcoholic caustic alkali (e. g., sodium hydroxide and an alcohol) at the boiling point of the mixture while refluxing at atmospheric pressure. Alkali metal hydroxide and an alcohol are used instead of the equivalent preformed alcoholate because of lower cost. Methanol is preferred as the alcohol in view of its relative cheapness. The reduction product, aside from a small amount of aniline, is axozybenzene; the alkali metal alcoholates are not sufiiciently strong reducing agents to carry the reduction beyond the azoxy stage, under the usual conditions.

in the past, it was necessary to carry out the further reduction of azoxybenzene to azobenzene, and of azobenzene to hydrazobenzene, with stronger reducing agents (such as zinc and alkali, which are most costly than the alcoholic caustic alkali reducing agents), and/ or by heating at more elevated temperatures and pressures (such as temperatures of 148 to 180 C. and pressures of or more atmospheres) which has the usual disadvantages of operation under pressure as well as requiring expensivepressure apparatus.

Thus, the production of aromatic azo and hydrazo compounds from aromatic nitro compounds and their intcr'me'cliate reduction products presents anumber of problems from the standpoint of commercial practice.

A primary object of the present invention is to provide improvements in the alkaline reduction of reducible aromatic nitrogen compounds ofthe type referred to above i. e., containiras a nuclear substituent, nitrogen in a reducible form at a higher stage of oxidation than the hydrazo stage), whereby the reducing power of metal alcoholate reducing agents and especially of alcoholic caustic alkali reducing agents i enhanced and other advantages are secured.

Gther objects of the present invention are to provide a process for the production of aromatic azo compounds in good yields by the reduction of aromatic nitro compounds, and their reduction products up to and including 2,765,301 Patented Oct. 2, 1956 azoxy compounds, with metal alcoholates under moderate reaction conditions and in simple apparatus; and to provide a process for the production of aromatic hydrazo' compounds by reduction of aromatic nitro compounds, and their reduction products up to and including azo compounds, with metal alcoholates, and especially an alkali metal hydroxide and a lower alcohol, under moderate conditions and in simple apparatus.

Additional objects in part will be obvious and in part will appear hereinafter.

According to the present invention, the foregoing objects are accomplished by carrying out the metal alcoholate reduction of reducible aromatic nitrogen compounds of the type referred to above in a reaction mixture in which one or more reduction promoters of a novel class have been incorporated.

The novel class of reduction promoters employed in accordance with the present invention is constituted of socalled addition products of naphthoquinones with so-called sulfiding compounds. (The term addition products is employed in the broad sense; it includes prodnets of various degrees of reaction between the naphthoquinones with the sulfiding compounds.) They are formed by reacting one or more naphthaquinones (which term includes the unsubstituted naphthoquinones and their nuclear-substituted derivatives) with one or more sulfiding compounds. compound denotes and includes hydrogen sulfide, its salts (monosulfides and hydrosulfides), polysulfides, thio sulfates, thiocarbonates, hydrosulfites, thiocyanates and related compounds which react with quinones to form reduction products containing, as a nuclear substitutent, sulfur to which is linked a cation, sulfur or a carbon atom (see German Patent 175,070).

I have discovered that the inclusion in the reaction mixture of a small amount of an addition product of a naphthoquinone with a sulfiding compound, such as the addition product of 1,2- or 1,4-naphthoquinone with an alkali metal monosulfide, hydrosulfide, or polysulfide, has a modifying effect upon the reduction, as a result of which a number or" benefits may be secured.

Thus, as compared with a reduction carried out under the same conditions but in the absence of the addition product, the speed of the reduction is increased and/0r products of a higher stage of reduction are obtained, without substantial sacrifice of the total yield of reduction products secured from the starting material. In the reduction of aromatic nitro compounds, the inclusion of such addition product in the metal alcoholate reaction mixture makes possible the direct obtainment of azo and hydrazo compounds, without requiring the use of drastic operating conditions, such as high temperatures and pressures of 10 or more atmospheres. Similarly, the inclusion in the reaction mixture of such addition product makes possible the production of hydrazo compounds from nitro, azoxy and azo compounds by mean of metal alcoholate reducing agents without requiring the use of drastic operating conditions. By including such addition products in the reduction reaction mixtures, side reactions leading to the evolution of hydrogen gas during the reduction are suppressed almost completely, thereby greatly increasing the safety of the reduction process and minimizing. waste of the reducing agent.

Where no substantial increase in the degree of reduction is desired, the inclusion of an addition product of said type in the reduction reaction medium makes possible the use of milder reaction conditions or the use of vantages of being generally more active of being more As employed herein, the term sulfiding.

readily soluble or dispersible in the reaction media than the naphthoquinones, and of forming so small an amount, if any, of insoluble dark-colored byproducts in the reduction as not to be of consequence.

, The reduction promoters of the present invention can be obtained in various ways. Thus, they can be formed by mixing the naphthoquinone with the sulfiding compound in an inert solvent or diluent, such as an alcohol, water or a hydrocarbon solvent (e. g., benzene), and heating to the extent necessary. Ordinarily, the sulfiding compound is added to a solution or suspension of the naphthoquinone in the solvent or diluent at room temperature, the sulfiding'compound employed being one having suflicient solubility in the solvent or diluent to permit it to react with the naphthoquinone. In some cases (for example, when reacting a naphthoquinone with an excess of an alkali metal monosulfide, hydro'sulfide, or polysulfide), the reaction takes place spontaneously. In some other cases, gentle heating may be required to initiate the reaction. In still other cases (for example sodium thiosulfate and potassium thiocyanate) an acid, e. g. acetic and/or hydrochloric acid, is added to initiate or promote reaction between the sulfiding compound and the naphthoquinone. Usually, the reaction is brought to substantial completion by heating the reaction mixture. Preferably, the formation of the reduction promoter is carried out in an alcohol as the reaction medium, and especially the alcohol to be employed in the reduction, with a sulfiding compound soluble in the alcohol. Besides the greater sirnplicity and convenience afforded by this procedure, since the resulting solution can be used'as such for the promotion of the reduction by incorporating it in the reduction reaction mixture, the reduction promoters thus obtained generally possess the advantage of superior reduction-promoting activity. If desired, however, the reduction promoter can be separately preparedas by forming it in a solvent or diluent (such as an alcohol, water, benzene, etc.) in the manner described above, and evaporating the resulting reaction mixture to dryness, or by acidifying the diluted reaction mixture and recovering the resulting precipitate-and then can be incorporated into the reaction mixture, preferably by dissolving or suspending it in the alcohol to be employed in the reduction.

The relative proportions of naphthoquinone and of sulfiding compound employed in forming the reduction promoters of the present invention can be varied, depending upon the desired activity of the promoter. In general, an amount of sulfiding compound somewhat in excess of an equimolar proportion is employed so as to convert all of the naphthoquinone to an addition product, and in the case of alkaline salts of hydrogen sulfides (e. g., an alkali metal or ammonium monosulfide, hydrosulfide or polysulfide) reduction promoters of excellent activity are obtained when about 1.4 mol of the salt per mol of naphthoquinone is employed. Larger proportions of said alkaline salts do not materially increase the activity of the reduction promoters; proportions substantially smaller than equimolar lead to reduction promoters of correspondingly decreased activity.

Naphthoquinones which may be employed in forming the reduction promoters of the present invention are preferably those in which a position ortho or para to a carbonyl group is unsubstituted or is occupied by a halogen atom. Examples of suitable naphthoquinones are: 1,4- naphthoquinone; 1,2-naphthoquinone; 2,6-naphthoquinone; 2,3-dichloro-1,4-naphthoquinone; 2-hydroxy-1,4-

naphthoquinone; 2-methyl-1,4-naphthoquinone; Z-nitro- 1,,4-naphthoquinone; 4-chloro 6 bromo l methyl-2,3- naphthoquinone and 5,8-dihydroxy-l,4-naphthoquinone.

Examples of sulfiding compounds which may be employed in forming the reduction promoters of the present invention are: hydrogen sulfide and salts of hydrogen sulfides (including monosulfides, hydrosulfides and polysiilfides), as well as thiocarbonates, thiosulfates, hydrosulfites, thiocyanates, and related compounds of the type referred to above, in whichthe cation is a metal such as zinc or a light metal, and especially salts having an alkaliforming cation (that is, an alkali-forming metal or ammonium or an organic ammonium radical); they include inorganic salts, such as the sodium, potassium, lithium, calcium, magnesium, zinc and ammonium salts, and organic ammonium salts derived from organic bases, such as mono-, di-, and trimethyl-, ethyl-, propyl-, butyl-, amyl-, hexyl-, benzyland ethanol-amines, and diethylhydroxyethyl-amine.

Addition products preferred for use as reduction promoters in the reduction of reducible aromatic nitrogen compounds by means of metal alcoholates are the addition products of naphthoquinones, and especially of 1,4- naphthoquinone, with salts of hydrogen sulfides (including monosulfides, hydrosulfides and polysulfides) and with thiosulfates, particularly salts having alkali-forming cations (including organic amine salts) and especially alkali metal salts of hydrogen sulfide (including ammonium salts). Addition products of 1,4-naphthoquinone with sodium sulfide, ammonium sulfide, sodium hydrosulfide and ammonium hydrosulfide are especially preferred in view of their outstanding activity as reduction promoters, coupled with their availability and relatively low cost.

The exact nature of the addition products which function as reduction promoters in accordance with the present invention is not known. In general, the products resulting 'rom reaction of the naphthoquinones with the sulfiding compounds contain sulfur in molecular combination. On the basis of German Patent 175,070 and Journal f. prakt. Chemie [2], vol. 108 (1924), page 273, the addition products would seem to be mercaptan, sulfide, thiosulfonate, or like sulfur derivatives of reduction products of the naphthoquinones. They may, however, be other forms of sulfur-containing addition products or reaction products of the naphthoquinones and/or they may be converted to these or other products when in corporated into the reduction reaction mixtures. Accordingly, the invention is not limited by any theoretical considerations, but includes various products resulting from the interaction of naphthoquinones with the sulfiding compounds.

In the practice of the present invention, the reducible aromatic nitrogen compound is subjected to the reducing action of a metal alcoholate reducing agent in a reaction mixture in which one or more addition products of a naphthoquinone with a sulfiding compound have been incorporated a reduction promoter.

The addition product or products may be incorporated in the reaction mixture in various ways and at various times without departing from the scope of the invention. Thus, they can be preformed separately and then added to the reaction mixture, preferably in the form of a solution or suspension in an alcohol; or they can be formed in the reaction mixture, preferably in an alcohol of the type to be employed in the reduction. In the preferred practice of the invention, wherein the reducible aromatic nitrogen compound is heated with a caustic alkali and an alcohol (preferably sodium hydroxide and methanol) at the boiling point of the reaction mixture, the addition product is incorporated prior to the addition of the alkali and preferably is mixed with (or formed in) the alcohol, which is then mixed with the caustic alkali and heated, before the nitrogen compound to be reduced is added to the mixture.

The addition products can be employed in various amounts, although it is a feature of the present invention that only small amounts thereof are required. Thus, amounts ranging from 15 to ,6 mol of addition product per mol of reducible aromatic nitrogen compound may be employed. The minimum amount required to produce a significant reduction-promoting effect varies with the individual addition product, the nature of the reductible aromatic nitrogen compound, and the reaction conditions. In general, a greater reduction-promoting efiect is secured by increasing the amount of reduction promoter employed and a lesser effect results from decreasing the amount employed, other reaction conditions being constant. Ordinarily, amounts greater than A mol of adidtion product per mol of reducible aromatic nitrogen compound are not advantageous, although they may be used if desired, since the additional benefits derived therefrom are not of commercial importance and hence the increased cost of the extra amount of addition product is not sufficiently compensated.

The invention will be illustrated by the following specific examples, but it is to be understood that it is not limited to the details thereof and that changes may be made without departing from the scope of the invention. The temperatures are in degrees centigrade and the parts and percentages are by weight, unless designated as parts by volume. Where parts are by volume, the amount signifies the volume occupied by the same number of parts by weight of water at 4 C. The naphthoquinones employed were technical products, mainly containing water as impurity; the quantities thereof referred to in the examples are the corresponding amounts on substantially 100% naphthoquinone basis. In all cases, a yield of reduction products (oily layer) corresponding with about 98 percent of the theoretical was obtained, based on the nitroaromatic starting material.

EXAMPLE 1 Part 1.A methanol solution of a reduction promoter of high activity was prepared by adding 14 parts of sodium hydrosulfide (containing 70% of NaHS) to a solution of 20.0 parts of 1,4-naphthoquinone in 400 parts by volume of methanol at 25. The resulting mixture, whose temperature rapidly rose spontaneously to 40, was heated to boiling and refluxed for 2 hours to complete the reaction. The 1,4-naphthoquinone initially was substantially undissolved but passed into solution completely during the reaction. A deep brown solution was formed. The mass was then cooled and diluted with methanol to 500 parts by volume.

Part 2.-Two hundred and thirty-four parts by volume of the resulting methanol solution of reduction promoter (containing an amount of reduction promoter corresponding with 9.5 parts of 1,4-naphthoquinone), 316 parts by volume of methanol, 0.1 part of a keryl benzene sodium .sulfonate dispersing agent (Nacconol NR) and 0.1 part of an alkyl naphthalene sodium sulfonate wetting agent (Naccosol A) were charged to a flask equipped with a reflux condenser, agitator, dropping funnel and thermometer. Five hundred and fifty parts of sodium hydroxide flakes were then added over the course of about 15 minutes while maintaining the flask contents at a temperature below 55 by suitable cooling. The resulting mixture was heated to refluxing (95), 838 parts of nitrobenzene were added over the course of about 2 hours, while cooling when necessary to prevent excessively vigorous reaction and reflux, and the mixture thus obtained was boiled and refluxed atatmospheric pressure for hours. Unreacted methanol was then removed by distillation until azobenzene appeared in the distillate, while maintaining the volume of the mixture substantially constant by addition of 450 parts by volume of water. The mixture was allowed to stand and separate into an upper oil phase, containing the reduction product, and a lower aqueous phase. The aqueous phase, consisting essentially of sodium hydroxide and sodium formate in solution, was drawn off at about 100. The remaining molten oil was analyzed for its content of azobenzene, hydrazobenzene and aniline by a modification of the analytical method described in P. B. Report No. 70188, Frames 5965-5969. It consisted essentially of a mixture of azobenzene, hydrazobenzene and a small amount of aniline, of which 56.7% was azobenzene, 41.8% was hydrazobenzene and 1.5% was aniline.

The deep brown methanol solution of addition product formed in Part 1 of the above exampleturned a deep violet-red when exposed to .air. The addition product was precipitated from solution by acidification with dilute hydrochloric acid. The resulting precipitate, which was initially dull yellow but rapidly became dark green on exposure to air (when subjected to drying), redissolved in aqueous sodium hydroxide to give a deep brown solution similar in color to the original .solution. The precipitate did not melt when heated above 200. When dissolved in methanol and employed in a corresponding amount in the process of Part 2 of Example 1, .in place of the solution produced in Part 1 of said example, the promoter activity was somewhat less (the proportion of hydrazobenzene in the product was lower and the proportion of azobenzene was higher than in Part 2 of Example 1).

EXAMPLE 2 The process of Example 1 was repeated, with the substitution of 20 parts of 1,2-naphthoquinone for the 1,4- naphthoquinone therein employed and the use of 15 parts of 70% sodium hydrosulfide. The resulting product consisted essentially of 66% of azobenzene, 31% of hydrazobenzene and 3% of aniline.

EXAMPLE 3 Part 1.-A methanol solution of a reduction promoter of high activity was prepared by adding 18 parts of commercial sodium monosulfide (containing 60% NazS) to a solution of 20.0 parts of 1,4-naphthoquinone in 400 parts by volume of methanol at room temperature; agitating the resulting mixture, which warmed spontaneously, at about 30 for 1 hour, and then refluxing at atmospheric pres sure for 1 /2 hours. The 1,4-naphthoquinone initially was substantially undissolved but passed into solution completely during the reaction. A deep brown solution was formed. The mass was then cooled and diluted with methanol to 500 parts by volume.

Part 2.-The process of Example 1, Part 2, was repeated, employing as the reduction promoter 234 parts by volume of the methanol solution obtained in Part 1 of this example (containing an amount of reduction promoter corresponding with 9.5 parts of 1,4-naphthoquinone). The resulting product consisted essentially of 56.0% of azobenzene, 42.5% of hydrazobenzene and 1.5% of aniline.

EXAMPLE 4 Part 1.A mixture of 400 parts by volume of methanol, 16 parts of commercial sodium sulfide (containing 60% NazS) and 8 parts of sulfur was agitated at 25 to 30 until the sulfur was completely dissolved (1 hour) forming a solution of sodium polysulfide (NazSs); then 20.0 parts of 1,4-naphthoquinone were added. The mixture thus obtained, which became warm from the ensuing reaction, was agitated for a half hour at 30 to 35, and then refluxed for an hour and a half, during which time considerable H28 was evolved. The 1,4-naphthoquinone initially was substantially undissolved but passed into solution completely during the reaction. A deep brown solution was formed. The mass was then cooled and diluted with methanol to 500 parts by volume.

Part 2.-The process of Example 1, Part 2, was repeated, employing as the reduction promoter 234 parts by volume of the methanol solution obtained in Part 1 of this example (containing an amount of reduction promoter corresponding with 9.5 parts of 1,4-naphthoquinone). The resulting product consisted essentially of 61.7% of azobenzene, 36.8% of hydrazobene and 1.5 of aniline.

EXAMPLE 5 Part 1.A solution of parts of sodium thiosulfate (Na-zS2O3.5H2O) in parts of water and 40 parts by volume of 50% acetic acid was added to a slurry of parts of 1,4-naphthoquinone in 120 parts by volume of denatured ethyl alcohol (Formula 2B). The mixture was agitated for two hours at to After addition of 2 parts of sodium bisulfite the slurry was filtered to remove a small amount of a gray-green solid. The filtrate was salted with 75 parts of potassium chloride, and the resulting precipitate was filtered off, washed with potassium chloride solution, and dried at 50 in vacuo. The dried product, weighing 66 parts, was at first white and then slowly turned green. The dry product was charged to the methanol used in the reduction experiment.

Part 2.43.5 parts of the resulting dry product (corresponding with 9.4 parts of 1,4-naphthoquinone employed as starting material) were dissolved in 420 parts of methanol and the reduction was then carried out in the manner described in Example 1, Part 2. The resulting product contained 69.8% of azobenzene, 29.5% of hydrazobenzene and 0.7% of aniline.

The reduction process of Example 1, Part 2 was repeated with a number of additional reduction promoters. In each case a total of 420 parts of methanol (including that introduced with the reduction promoter), 550 parts of sodium hydroxide and 840 parts of nitrobenzene were used. The reduction promoters were prepared in the manner set out below in the following specific examples. In carrying out the reduction, 235 parts by volume of the methanol solution of addition product was added to the methanol used in forming the reduction reaction mixture (corresponding with 9.4 parts of naphthoquinone initially employed). The results obtained were as set out in the following examples.

EXAMPLE 6 To 400 parts by volume of methyl alcohol there were added first 6 parts of ammonia gas (bubbled in at 25 to 30), then 10 parts of hydrogen sulfide gas (bubbled in at 25 to 30). To the clear yellow solution of ammonium hydrosulfide thus obtained, 20 parts of 1,4- naphthoquinone were added and the mixture was refluxed for two hours, during which complete solution resulted. The resulting dark brown solution was cooled and diluted with methyl alcohol to 500 parts by volume. The reduction product consisted essentially of 65.2% of azobenzene, 34.0% of hydrazobenzene and 0.8% of aniline.

EXAMPLE 7 To a mixture of 20 parts of 1,4-naphthoquinone in 400 parts by volume of methyl alcohol, 1 part of sodium hydrosulfide (70% NaHS) and 6 parts of sulfur were added. The mixture was refluxed for six hours during which time a black solution was formed. The solution was cooled and diluted with methyl alcohol to 500 parts by volume. The reduction product consisted essentially of 66.1% of azobenzene, 32.5% of hydrazobenzene and 1.4% of aniline.

EXAMPLE 8 To 400 parts by volume of methyl alcohol, 20 parts of sodium sulfide (60% NazS) and 13 parts of carbon bisulfide were added. The mixture was refluxed for onehalf hour with agitation and allowed to col to about room temperature with agitation for one hour. Then 20 parts of 1,4-naphthoquinone were added, causing spontaneous warming and darkening of the solution. The mass was refluxed for two hours, cooled and diluted with methyl alcohol to 500 parts by volume. The reduction was similar to that obtained in Examples 6 and 7; it consisted essentially of 65% azobenzene, 34% of hydrazobenzene and 1% of aniline.

When the reduction process was carried out in the manner described in the above specific examples, but without addition of any reduction promoter, the product consisted essentialy of azoxybenzene (it had a setting point of 33.5 C.) and copious quantities of hydrogen were evolved.

As noted above, the invention is not limited to the details of the foregoing illustrative examples, and changes can be made without departing from the scope of the invention.

Thus, the process is applicable to the reduction of other aromatic nitrogen compounds containing nitrogen in a reducible form as a nuclear substituent, as for example, o-nitrotoluene, rn-nitrotoluene, o-nitrochlorobenzene, mnitrochlorobenzene, o-nitrophenetole, o-nitrobenzoic acid and o-nitrobenzene sulfonic acid. In view of the extensive use of hydrazobenzene and its o-substituted deriva tives (such as o,o-dichloro-hydrazobenzene, o,o'-hydrazotoluene, o,o-hydrazonanisole, o,o-diethoxy-hydrazobenzene, etc.) as intermediates for the manufacture of benzidine and related derivatives of benzidine, the process of the present invention is of special value as a means for reducing the cost of manufacturing such hydrazo compounds from the corresponding reducible mononuclear aromatic nitrogen compounds (such as, nitrobenzene and its o-substituted derivatives and reduction products thereof) in which the nitrogen is at a higher stage of oxidation than the hydrazo stage.

The reduction of aromatic nitro compounds to azoxy compounds (1), of azoxy compounds to azo compounds (2), and of azo compounds to hydrazo compounds (3) proceeds according to the following equations, in which R is an aromatic nucleus:

In carrying out the reduction by means of sodium hydroxide and methanol, it is preferable to employ these reagents in amounts in excess of those theoretically required. Extra methanol over that theoretically required is generally desirable for use as a solvent, and an additional excess is desirable to counteract the diluting efiect of the Water generated in accordance with above Equations 1 and 2, which would otherwise tend to retard the reaction. An excess of sodium hydroxide also is desirable since it tends to increase the rate of reaction.

It is possible to carry out the reduction of a particular reducible aromatic nitrogen compound to various stages, depending upon the amounts of sodium hydroxide and methanol, as well as the nature and amount of the particular reduction promoter employed. Thus, with the aid of the reduction promoters of the invention, it is possible to reduce nitrobenzene to hydrazobenzene in a single reaction mixture, as illustrated in the above examples. It is also possible to reduce nitrobenzene to azoxyand/ or azobenzene in one reaction mixture, and then reduce the resulting azoxybenzene and/or azobenzene to hydrazobenzene with a fresh charge of sodium hydroxide and methanol.

. The temperature at which the reaction is carried out also may be varied although, in the reduction performed with the aid of alcoholic caustic alkali, temperatures at or near the boiling point of the reaction mixture at atmospheric pressure (ordinarily about to are preferred. At lower temperatures, the reaction is slower, under otherwise similar conditions, and may require an excessively long time to produce the same results as the preferred temperatures. Conversely, higher reaction temperatures and pressures result in a short time cycle but require the use of closed reaction vessels. However, temperatures greatly exceeding 110, though not precluded, are less desirable; since even in the presence of the reduction promoters they lead to evolution of considerable amounts of hydrogen gas and formation of primary amines, with consequent loss of yield of the desired reduction products.

While for economical and simple operation it is preferred to use, as a solvent or diluent of the reaction mixture, an excess of the alcohol employed for the alcoholate, the invention is not limited thereto. Thus, other solvents and diluents can be employed; for example, the process may be carried out with amounts of sodium hydroxide and methanol only slightly in excess over the amounts theoretically required for the reduction in a reaction medium containing a sufficient amount of xylene to provide a stirrable reaction mass. Instead of xylene, other inert solvents or diluents may be used, such as benzene, toluene, monoand dichlorobenzenes. Further, while it is simpler to employ, as the solvent or diluent, an excess of the alcohol functioning as a reducing agent, other alcohols can be employed; also mixtures of alcohols can be used, especially where it is desirable to modify the boiling temperature of the reaction mixture.

As a matter of convenience and for economical operation, the process is generally carried out by forming a metal alcoholate in the reaction mixture; for example, by reacting caustic alkali with the alcohol. If desired, however, preformed metal alcoholates may be employed as reducing agents, in which case the diluting effect of the water formed as a by-product of the reaction of caustic alkali with the alcohol is avoided.

Sodium hydroxide and methanol are employed in the specific examples in view of their relatively lower cost and ready availability. The invention is not limited thereto, however, and other alkalis (for example, potas sium hydroxide) and other alcohols (for example, ethyl alcohol and the various propyl, butyl and higher alcohols) may be employed, if desired.

Other sulfiding compounds may be employed in place of those used in the above specific examples. Thus, addition products having e'l'tective reduction promoting action can be obtained by substituting corresponding amounts of hydrogen sulfide, calcium hydrosulfide, sodium hydrosulfite, or others of the sulfur compounds disclosed above, for the sulfur compounds employed in forming the addition products of the above specific examples.

The products of the reduction can be isolated from the reaction mixtures in any suitable manner. Ordinarily, the isolation of the reduction products can be carried out in the manner customarily employed in the absence of a reduction promoter. Thus, the reaction mixture may be cooled to crystallize the reduction product and filtered, and the cake washed with water to remove alcohol, sodium formate formed as a by-product of the reduction, and sodium hydroxide. Generally, it is preferred to steam distill the methanol (and dehydrate the aqueous methanol thus obtained, by fractional distillation for reuse in subsequent reactions) and then cool the remaining hot aqueous mass to crystallize the reduction product, which may be washed as usual with water. Where the product is molten in the hot mixture, as in the case of azoxybenzene, azobenzene, and mixtures of azobenzene and hydrazobenzene, it is simpler to stratify the mass into an aqueous phase and an oil phase, whereupon the latter can be readily separated as illustrated in the examples.

The Dy-products resulting from use of the reduction promoters of the present invention are generally soluble in the aqueous and/ or alcoholic layer noted above, and thus can be separated from the reduction products. When the use of a reduction promoter produces a small amount of insoluble by-product, it may be removed in any suitable manner, as by filtering the hot mixture prior to the phase-separation.

I claim:

1. The improvement in the method of reducing a reducible aromatic nitrogen compound selected from the group consisting of nitrobenzene, its ortho-methyl, halogen, methoxy, ethoxy, carboxy and sulfo derivatives, and their reduction products in which the nitrogen is at a higher stage of oxidation than the hydrazo stage, by the action of a metal alcoholate, which comprises carrying out the reduction in a reaction mixture in which has been incorporated an addition product of an unsubstituted naphthoquinone with a sulfiding compound.

2. A method as defined in claim 1 wherein the naphthoquinone is 1,4-naphthoquinone.

3. A method as defined in claim 2 wherein the cation of the salt is an alkali forming cation.

4. A method as defined in claim 2 wherein the cation of the salt is an alkali metal.

5. The improvement in the method of reducing a reducible aromatic nitrogen compound selected from the group consisting of nitrobenzene, its ortho-methyl, halogen, methoxy, ethoxy, carboxy and sulfo derivatives, and their reduction products in which the nitrogen is at a higher stage of oxidation than the hydrazo stage, by the action of a metal alcoholate, which comprises heating the reducible aromatic nitrogen compound with a reducing mixture of an alkali metal hydroxide and an alcohol in a reaction mixture in which has been incorporated a small amount of an addition product of an unsubstituted naphthoquinone with a sulfiding compound selected from the group consisting of a salt of a hydrogen sulfide, a thiosulfate and a thiocarbonate.

6. The improvement in the method of reducing a reducible aromatic nitrogen compound selected from the group consisting of nitrobenzene, its ortho-methyl, halogen, methoxy, ethoxy, carboxy and sulfo derivatives, and their reduction products in which the nitrogen is at a higher stage of oxidation than the hydrazo stage, by the action of a metal alcoholate, which comprises heating the reducible aromatic nitrogen compound with a reducing mixture of an alkali metal hydroxide and a lower alcohol in a reaction mixture in which has been incorporated an addition product of 1,4-naphthoquinone with a sulfiding compound soluble in the alcohol.

7. The improvement in the method of reducing a reducible aromatic nitrogen compound selected from the group consisting of nitrobenzene, its ortho-methyl, halogen, methoxy, ethoxy, carboxy and sulfo derivatives, and their reduction products in which the nitrogen is at a higher stage of oxidation than the hydrazo stage, by the action of a metal alcoholate, which comprises heating the reducible aromatic nitrogen compound with a mixture of sodium hydroxide and methanol in a reaction mixture in which has been incorporated a small amount of an addition product of 1,4-naphthoquinone with a sulfiding compound selected from the group consisting of a salt of hydrogen sulfide, a thiosulfate and a thiocarbonate.

8. The improvement in the method of reducing a reducible aromatic nitrogen compound selected from the group consisting of nitrobenzene, its ortho-methyl, halogen, methoxy, ethoxy, carboxy and sulfo derivatives, and their reduction products in which the nitrogen is at a higher stage of oxidation than the hydrazo stage, by the action of a metal alcoholate, which comprises heating the reducible aromatic nitrogen compound with a reducing mixture of sodium hydroxide and a lower alcohol in a reaction mixture in which has been incorporated an addition product of an unsubstituted naphthoquinone with a salt of a hydrogen sulfide.

9. A method as defined in claim 8, wherein the alcohol is methanol and the sulfiding compound is a hydrosulfide having an alkali-forming cation.

10. A method as defined in claim 8, wherein the alcohol is methanol and the sulfiding compound is a monosulfide having an alkali-forming cation.

11. A method as defined in claim 8, wherein the al 11 cohol. is methanol and the sulfiding compound is a polysulfide having an alkali-forming cation.

12. A method as defined in claim 8, wherein the alcohol is methanol and the sulfiding compound is a thiosulfate having an alkali-forming cation.

13. A method as defined in claim wherein the addition product has been formed by reacting an unsubstituted naphthoquinone with an alkali metal salt of a hydrogen sulfide in an alcoholic medium.

14. A method as defined in claim 5 wherein the addition product has been formed by reacting an unsubstituted naphthoquinone with an alkali metal thiosulfate in an alcoholic medium.

15. A method as defined in claim 8 wherein the addition product is formed by reacting 1,4-naphthoquinone with an alkali metal salt of a hydrogen sulfide, the resulting addition product is incorporated in a mixture of methanol and sodium hydroxide, and the reducible aromatic nitrogen compound is heated with the resulting mixture.

16. A method as defined in claim 15, wherein the alkali metal salt of a hydrogen sulfide is sodium monosulfide, the sodium hydroxide is admixed with a solution of the addition product in methanol, and nitrobenzene is heated with the resulting mixture.

17. A method as defined in claim 15, wherein the alkali metal salt of a hydrogen sulfide is sodium hydrosulfide, the sodium hydroxide is admixed with a solution of the addition product in methanol, and nitrobenzene is heated with the resulting mixture.

18. A method as defined in claim 8 wherein the addition product is formed by reacting 1,4-naphthoquinone 12 with an alkali metal thiosulfate, the resulting addition product is incorporated in a mixture of methanol and sodium hydroxide, and the reducible aromatic nitrogen compound is heated With'the resulting mixture.

19. A method as defined in claim 5 wherein the addition product is formed by reacting 1,4-naphthoquinone with a sodium sulfide in methanol, the resulting addition product is incorporated in a mixture of methanol and sodium hydroxide, and nitrobenzene is heated with the resulting mixture.

20. A method as defined in claim 5 wherein the addition product is formed by reacting 1,4-naphthoquinone with an ammonium sulfide in methanol, the resulting addition product is incorporated in a mixture of methanol and sodium hydroxide, and nitrobenzene is heated with the resulting mixture.

References Cited in the file of this patent UNITED STATES PATENTS 2,344,244 Freed et a1 Mar. 14, 1944 2,486,358 Hallie Oct. 25, 1949 2,549,118 Newby Apr. 17, 1951 2,551,003 Johnson May 1, 1951 2,600,000 Kamlet June 10, 1952 2,645,636 Sogn July 14, 1953 FOREIGN PATENTS 175,070 Germany May 26, 1905 OTHER REFERENCES Fieser et al.: Chem. Abst., vol. 29 (1935), p. 2950. 

1. THE IMPROVEMENT IN THE METHOD OF REDUCING A REDUCIBLE AROMATIC NITROGEN COMPOUND SELECTED FROM THE GROUP CONSISTING OF NITROBENZENE, ITS ORTHO-METHYL, HALOGEN, METHOXY, EHTOXY, CARBOXY AND SULFO DERIVATIVES, AND THEIR REDUCTION PRDUCTS IN WHICH THE NITROGEN IS AT A HIGHER STAGE OF OXIDATION THAN THE HYDRAZO STAGE, BY THE ACTION OF A METAL ALCOHOLATE, WHICH COMPRISES CARRYING OUT THE REDUCTION IN A REACTION MIXTURE IN WHICH HAS BEEN INCORPORATED AN ADDITION PRODUCT OF AN UNSUBSTITUTED NAPHTHOQUINONE WITH A SULFIDING COMPOUND. 